Normally, a block sync signal is added to the head of a data block going to be transmitted or recorded. At the time of data reception or reading, the sync signal is detected to recognize the head of the block. The “block” referred to herein is a unit consisting of a plurality of data words or code words. For example, a block in DAT (digital audio tape) is composed of 36 symbols in total including 2 symbols of ID (identity) data, 1 symbol of ID parity and 32 symbols of data word. One symbol is of 8 bits before encoded with an eight-to-ten (8/10) code that is a recording code while it is of 10 bits after encoded with the recording code. For recording, each 8-bit symbol forming a block is first encoded with the 8/10 code and then a sync signal (sync word) of 1 symbol (10 bits), not existent in the sequence of the 8/10 code, is added to the head of the block. For reading, the head of the block is recognized by detecting the sync word and then the encoded ID data, parity and 32-symbol data word are decoded symbol by symbol. The 8/10 code is intended for use with data recorded by the NRZI (non-return-to-zero-inverted) recording method. However, a maximum length of a succession of same bits in an NRZI-converted sequence (will be referred to as “Tmax” hereunder) is 4 and Tmax will not continuously repeat more than 2 times. A sync word includes a pattern in which Tmax not appearing in any code sequence continuously repeated 2 times, whereby the sync word can be distinguished from the code sequence.
Recently, the TCPR (trellis-coded partial response) method has been studied actively in the field of magnetic recording. This method is to take a partial response characteristic and code characteristic in combination with each other to increase the Euclidian distance between transmission-channel (recording/reading channel) output sequences (free squared Euclidian distance: d2free). An increase in d2free is equivalent to an increase in signal level. Therefore, the TCPR method improves SNR (signal-to-noise ratio) of data when detected. The code used in the TCPR method is generally called “trellis code”.
Note here that the extended partial response Class 4 (will be referred to as “EPR4” hereinafter) normally used as a partial response provides a method of equalizing waveforms equalized for unit-pulse response to each other to be (1, 1, −1, −1) at a sampling point (where a symbol exists). The system polynominal equation of EPR4 is expressed like a following formula (1):G(D)=(1−D)(1+D)2  (1)where D is a 1-bit delay operator.
A state transition diagram of EPR4 is shown in FIG. 12, and a trellis diagram of EPR4 is shown in FIG. 13. The minimum Euclidian distance d2free indicated with a doublet in the trellis diagram of EPR4 is:d2free=((+1)−(0))2+((+1)−(0))2+((−1)−(0))2+((−1)−(0))2=4
In the trellis-coded extended partial response Class 4 (will be referred to as “TCEPR4” hereinafter) in which the maximum-likelihood decoding is done with the use of EPR4 and constraints of modulation code, the squared Euclidian distance d2free can be increased from 4 to 6 by constraining the 16/18 code and thus a Viterbi detection gain of 1.8 dB can be gained for EPR4.
Since a Viterbi decoder for TCEPR4 operates at every predetermined 18 bits, even a phase lag of 1 bit will result in decoding a quite different bit.
Therefore, to reset the TCEPR4 Viterbi decoder at every predetermined 18 bits, a synchronization has to be detected. The synchronization detection itself has to be made by an EPR4 Viterbi decoder inferior by 1.8 dB in detection gain.
Note here that the accuracy of block sync signal detection will greatly influence the quality of data reception or reading. For example, if no sync signal can be detected due to any disturbance, all data will be lost. Also, if a sync signal is erroneously detected, all data will be lost for a period from the erroneous detection until a sync signal is correctly detected. Further, in the DAT, for example, if a sync word is erroneously detected, ID data will also be detected erroneously. ID data includes address information, and if it is written to a memory with the use of an erroneously detected address, data in other blocks will also possibly be destroyed.